1. Field of the Invention
This invention relates to an improved heat spreader for use with integrated circuit packages and in particular to an improved heat spreader for use with plastic integrated circuit packages.
2. Prior Art
As semiconductor production volumes have increased, more cost effective packages for integrated circuit chips have been developed. In particular, the plastic integrated circuit package, such as the ball grid array package, described in U.S. Pat. No. 5,241,133, has proven to be an acceptable type of integrated circuit package. Plastic integrated circuit packages have several advantages including: low production costs, increased input/output lead capabilities, and small size. However, plastic integrated circuit packages provide relatively poor heat dissipation from the integrated circuit chip to the environment or ambient outside the package. This is because plastic, unlike the materials used in older packaging techniques such as metal or ceramics, is thermally insulative and tends-to trap the heat generated by the integrated circuit chip within the package itself.
The inability of plastic integrated circuit packages to efficiently dissipate heat is a particular problem since new generations of integrated circuit chips are high power devices that generate considerable heat. This heat can adversely effect the performance of the integrated circuit chip, and even destroy the integrated circuit chip, if the generated heat is not effectively conducted away from the integrated circuit chip to the outside environment. Therefore, unless measures are taken to increase the heat dissipation capabilities of plastic packages, this otherwise highly advantageous form of integrated circuit chip packaging can be unsuitable for new generations of integrated circuit chips.
Some plastic packages have been fitted with a heat spreader in an effort to improve heat dissipation. FIG. 1A is a cross-sectional side view of a ball grid array package 100 with a prior art heat spreader 102 made of a flat sheet of metal. Ball grid array 100 includes a substrate 104 with insulation layers 106, conductive trace layers 108, and cavity 116. An integrated circuit chip 114 is positioned in the bottom of cavity 116. Bond wires 118 make the electrical connections between bond pads (not shown) on integrated circuit chip 114 and a first conductive trace layer 108a on substrate 104. (In an alternative embodiment, the integrated circuit chip is mounted in "flip chip" or "C4" configuration without the use of bond wires 118. Flip-chip mounting of integrated circuits is well known in the art. Therefore, a detailed description of this method is omitted).
Vias or conductive through holes 122 are used to electrically connect conductive trace layer 108a with the other conductive trace layers 108 and solder balls 110. Solder balls 110 are then used to make electrical connection between ball grid array 100 and a next level electrical structure (not shown) such as a printed circuit board.
Once integrated circuit chip 114 is positioned in cavity 116, and electrical connections are made, cavity 116 filled with encapsulant 120. Typically, encapsulant 120 is in liquid form when it is applied in cavity 116. After application, encapsulant 120 is cured and solidifies to form a protective case around integrated circuit chip 114. Encapsulant 120 is typically an epoxy base resin.
Once encapsulant 120 solidifies, prior art heat spreader 102 is attached to prior art ball grid array 100 with adhesive layer 112. FIG. 1B is a plan view of the lower surface 102a of prior art heat spreader 102. FIG. 1C is an perspective side view of prior art heat spreader 102 attached to ball grid array 100. Typically, lower surface 102a of heat spreader 102 is affixed to the upper surface 104a of substrate 104 and the upper surface 120a of encapsulant 120 with adhesive layer 112.
FIG. 2 is a cross-sectional side view of a second prior art ball grid array package 200 with heat spreader 102. In the embodiment of FIG. 2, cavity 216 is created by the application of a plastic over-mold 206 which covers sides 208 and partially covers upper surface 204a of substrate 204. Once created, cavity 216 is filled with encapsulant 120 as described above with respect to prior art ball grid array 100. Heat spreader 102 is then applied to upper surface 120a of encapsulant 120 with adhesive layer 112 as also described above.
To allow optimum heat dissipation, the heat generated by integrated circuit chip 114 should be conducted as directly as possible from integrated circuit chip 114, through encapsulant 120, to heat spreader 102 which then radiates the heat to the external environment. However, with prior art ball grid arrays 100 and 200, adhesive layer 112 represents an additional, and possibly thermally insulative, layer between integrated circuit chip 114 and heat spreader 102. Therefore, with prior art ball grid arrays 100 and 200, heat is not dissipated as efficiently as possible and the electrical performance of integrated circuit chip 114 can suffer. Further, adhesive layer 112 represents additional production cost, both in terms of materials used and the time and equipment necessary apply those materials.
Attempts have been made to eliminate adhesive layer 112 by applying prior art heat spreader 102 directly to upper surface 120a of encapsulant 120 while encapsulant 120 is still in liquid form. Lower surface 102a of heat spreader 102 then bonds directly with upper surface 120a of encapsulant 120 as encapsulant 120 solidifies. In theory, the adhesion takes place without using any additional adhesive. However, this method has proven ineffective for use with prior art heat spreader 102 for two reasons. First, the continuous, and typically smooth, lower surface 102a of prior art heat spreader 102 (see FIG. 1B) makes the bond between heat spreader 102 and surface 120a of encapsulant 120 unreliable. As a result, prior art heat spreader 102 can separate from the plastic integrated circuit package. Second, as described in detail below, applying prior art heat spreader 102 directly to encapsulant 120 often causes air bubbles to form in encapsulant 120. These air bubbles create serious problems with respect to circuit performance and package reliability.
FIG. 3 is a cross-sectional side view of a prior art ball grid array 300 with prior art heat spreader 102 applied directly to upper surface 120a of encapsulant 120. FIG. 3 shows air bubbles 302 which often form when prior art heat spreader 102 is applied directly to surface 120a of encapsulant 120.
Air bubbles 302 create several problems. First, the air in air bubbles 302 can become a thermally insulative layer between integrated circuit chip 314 and heat spreader 102 (see for example air bubble 302a in FIG. 3). The additional thermal insulation created by air bubbles 302 defeats the purpose of applying heat spreader 102 directly to encapsulant 120 in the first place, i.e., to decrease insulation between integrated circuit chip 314 and heat spreader 102 by eliminating adhesive layer 112 (see FIG. 1A).
In addition, air bubbles 302 can form in, or move to, the areas 314 within cavity 316 surrounding bond wires 118. As a result, bond wires 118 lose the support provided by encapsulant 120 and break more easily, causing the integrated circuit to fail.
Further, contaminants, such as moisture, can be trapped in air bubbles 302. These contaminants can compromise package integrity and adversely affect the performance and reliability of integrated circuit chip 314.
Additionally, the heat generated by integrated circuit chip 314 can cause the air trapped in air bubbles 302 to expand. As the air expands, it can put pressure on encapsulant 120 and create stress cracks in the package which further compromise package integrity.
As described above, it is highly desirable to avoid forming air bubbles 302 in encapsulant 120. Air bubbles 302 form in encapsulant 120 because prior art heat spreaders 102 are typically a uniform and continuous metal sheet or plate (see FIGS. 1B and 1C). As a result of their uniform structure, when prior art heat spreaders 102 are initially placed directly on surface 120a of encapsulant 120, air between the lower surface 102a of heat spreader 102 and upper surface 120a of encapsulant 120 is forced down into encapsulant 120 (recall that encapsulant 120 is, at this point in the manufacturing process, in liquid form). The high viscosity of encapsulant 120 in its liquid form often prevents air bubbles 302 from escaping to the sides 310 of cavity 116 before encapsulant 120 cures and hardens. As a result, when encapsulant 120 hardens, air bubbles 302 are permanently formed in cavity 116.
Several unsuccessful attempts have been made to resolve the air bubble problem. For instance, dimples or indentations (not shown) have been added to the lower surface 102a of heat spreader 102 to allow the air to escape before heat spreader 102 settles into position. In another attempt, heat spreader 102 has been given a negative draft to force air bubbles 302 to the side and out of cavity 316. However, due to the high viscosity of encapsulant 120 in its liquid state, these attempts to solve the problem have proven ineffective and manufacturers of integrated circuit chip packages have typically been forced to attach prior art heat spreader 102 to the integrated circuit package with adhesive layer 112.
What has been sought is a heat spreader which can be applied directly to the surface of an encapsulant, when the encapsulant is in a liquid state, so that the heat spreader is secured to a surface of encapsulant without the use of adhesives and without forming air bubbles within the encapsulant.